The present invention relates to a method of forming a resist film on a surface of the substrate or on a layer formed on a substrate such as a semiconductor layer, an insulating layer or an electrode layer.
In a photolithography process, a photoresist film is formed on a semiconductor wafer or a glass substrate for a liquid crystal display device (LCD substrate), followed by exposing the photoresist film to light in a predetermined pattern and subsequently developing the patterned photoresist film. Where the photoresist film is formed on the substrate, employed is a spin coating method disclosed in, for example, U.S. Pat. No. 5,002,008 or U.S. Pat. No. 5,688,322.
In recent years, the semiconductor wafer or the LCD substrate tends to be enlarged, with the line width of the circuit pattern becoming finer and finer. As a result, the photoresist film is required to be made thinner. The wafer size and the pattern line width have undergone various changes in the manufacture of a semiconductor device. Specifically, the pattern line width was 0.5 to 0.8 .mu.m in a 6-inch wafer. In this case, the corresponding resist film was about 1.0 .mu.m thick. Also, the pattern line width was 0.3 to 0.2 .mu.m in an 8-inch wafer. In this case, the corresponding resist film was about 0.8 to 1.0 .mu.m thick. Further, the pattern line width was 0.18 to 0.22 .mu.m in a 12-inch wafer. In this case, the corresponding resist film is considered to be about 0.3 to 0.5 .mu.m thick.
The thickness of the resist film formed by the spin coating method is greatly dependent on the rotating speed of the substrate and on the viscosity (concentration and temperature) of the resist solution. In order to make the resist film thinner, it is necessary to lower the viscosity of the resist solution and to increase the rotating speed of the substrate. For example, with change in the wafer diameter from 6 inches to 8 inches, the viscosity of the resist solution is lowered from 10 cP to 5 cP. Also, the wafer rotating speed is increased from, for example, 2,000 rpm to 3,000 rpm.
In a 12-inch wafer which is expected to be put to a practical use in future, the thickness of the resist film is to be decreased to below 0.5 .mu.m, making it necessary to further lower the viscosity of the resist solution and to further increase the wafer rotation speed. However, the lowest viscosity of the resist solution which can be used in the manufacture of a semiconductor device is about 3 cP. In order to form a resist film in a thickness of about 0.5 .mu.m by using a resist solution having a viscosity of about 3 cP, it is necessary to set the wafer rotation speed at about 4,000 rpm. However, the resist film formed by a spin coating method with such a high rotation speed is greatly non-uniform in thickness. Also, an excessively large load is applied to the spin chuck driving mechanism for rotating a large substrate at such a high speed, with the result that the spin chuck driving mechanism is required to be put to maintenance frequently. It follows that it is difficult to carry out a stable treatment uniformly.